From what I've gathers so far, a lifehack is not:

1. The only solution to a problem.
2. The common solution solution to a problem.

I'll update this as I find more. Feel free to add and debate.

From what I've gathers so far, a lifehack is a an unconventional solution to a problem.

So the opposite of that would be the conventional solution to a problem. A lifehack can't be the solution that's commonly used for that problem.

Likewise, a problem with only one solution can't be a lifehack, because if it only has a single solution then that solution is the conventional solution by default.